A horizontal type electroplating apparatus has widely been applied for electrogalvanizing purposes because of the merits thereof such as a smaller loss of voltage and the necessity of a lower strength of a conductor roll and a supporting roll as compared with a vertical type electro-plating apparatus. FIG. 3 is a schematic vertical sectional view illustrating a conventional horizontal type electro-plating apparatus used for electrogalvanizing a steel strip. As shown in FIG. 3, a conventional horizontal type electroplating apparatus comprises a plurality of horizontal type electroplating tanks 2, arranged in series, for receiving an electroplating solution 3 and a plurality of pairs of upper and lower anode plates 4 arranged in parallel with a steel strip 1 to be electroplated, which horizontally travels in the electroplating tanks 2, with the steel strip 1 therebetween. For the purpose of preventing flapping or loosening of the steel strip 1 caused by the long travelling distance of the steel strip 1 through the electroplating tank 2, a plurality of pairs of supporting rolls 8 are provided in the electroplating tanks 2 so as to pinch the steel strip 1 therebetween.
Near each of the inlet and the outlet for the steel strip 1 of the electroplating tank 2, a conductor roll 5 and a backup roll 6 are provided the former above the latter outside the electroplating tank 2 with the steel strip 1 therebetween. The steel strip 1 is electrically negatively charged by the conductor roll 5. An electrogalvanizing layer is formed on the surface of the steel strip 1 by the electrode reaction in the electroplating solution 3. To prevent the electroplating solution 3 from flowing to outside the electroplating tank 2 along with the steel strip 1, and to keep a constant level of the electroplating solution 3 in the electroplating tank 2, a pair of dam rolls 7 are provided one above the other at each of the inlet end and the outlet end of the electroplating tank 2 with the steel strip 1 therebetween.
Such a horizontal type electroplating apparatus is large in scale in general, comprising sequentially from 10 to 15 electroplating tanks 2 each having a length of 6 m, a width of 2.5 m and a depth of 1.0 m, thus requiring a very high cost for heating the electroplating solution 3 in a large quantity supplied into the electroplating tanks 2. Furthermore, since there is a long distance (1 m, for example) between the conductor roll 5 and the anode plates 4, partly because of the arrangement of the dam rolls 7 therebetween, resistance of the steel strip 1 itself causes a considerable loss of voltage.
With a view to overcoming these inconveniences, therefore, it is the recent tendency to try to reduce the scale of the horizontal type electroplating apparatus by switching over from the soluble electrode to the nonsoluble electrode or to reduce the distance between the conductor roll 5 and the anode plates 4 by eliminating the dam rolls 7.
In the case of a horizontal type electroplating apparatus without the dam rolls 7, a couple of the conductor roll 5 and the backup roll 6 are provided, instead of the pair of dam rolls 7, the former above the latter at each of the inlet end and the outlet end of the electroplating tank 2 with the steel strip 1 therebetween, and another couple of the conductor roll 5 and the backup roll 6 are provided also between the plurality of pairs of anode plates 4 in the electroplating tank 2. Since the lower portion of the conductor roll 5 is immersed into the electroplating solution 3, an electroplating metal is deposited onto the surface of the conductor roll 5 during electroplating. Deposition of the electroplating metal onto the surface of the conductor roll 5 results in an insufficient electrical contact between the conductor roll 5 and the steel strip 1, and partially hindered flow of electricity makes it impossible to achieve uniform electroplating. Furthermore, the electroplating metal deposited onto the surface of the conductor roll 5 is peeled off and falls down onto the steel strip 1, and as a result, the conductor roll 5 and the steel strip 1 bite the peeled electroplating metal therebetween. Consequently, flaws are produced on the surface of the steel strip 1, making the steel strip 1 a defective product. Such a defect in the product should absolutely be avoided. It is therefore necessary to prevent the electroplating metal from being deposited onto the surface of the conductor roll 5 or to remove the once deposited electroplating metal.
As a means to solve the above-mentioned problems, there is known an apparatus, disclosed in Japanese Patent provisional Publication No. 60-96,798 published on May 30, 1985, as shown in FIG. 4, in which a reverse-electrolyzing electrode 9 for electrolytically removing an electroplating metal deposited onto the surface of a conductor roll 5, the lower portion of which is immersed in an electroplating solution 3 in a horizontal type electroplating tank 2, is provided in the electroplating solution 3 near the conductor roll 5. In FIG. 4, 10 is an electric power source for reverse-electrolyzing, and 11 is an insulating cover for electrically shielding the surface of the reverse-electrolyzing electrode 9, except for the portion facing the conductor roll 5, from a plurality of pairs of anode plates 4 and a steel strip 1. According to the above-mentioned conventional apparatus, it is possible to electrolytically remove the electroplating metal deposited onto the surface of the rotating conductor roll 5.
The above-mentioned conventional apparatus has however the following problem: While it is necessary to accurately determine the timing of the start and the end of the electrolytic removal of the electroplating metal deposited onto the surface of the conductor roll 5, this determination has conventionally been accomplished through visual inspection by an operator, and the deposition of the electroplating metal occurs non-uniformly in the axial direction of the conductor roll 5. It is therefore difficult to accurately determine the timing of the start and the end of the electrolytic removal of the deposited metal. As a result, an early start of the electrolytic removal or a delayed end of the electrolytic removal of the deposited metal would result in electrolysis of the very surface of the conductor roll 5 not having an electroplating metal deposited thereon. This not only erodes the conductor roll itself, but also the metal, which is dissolved out from the conductor roll 5, is mixed as impurities into the electroplating solution 3. These problems occur also when the electroplating metal is not uniformly deposited over the entire surface of the conductor roll 5 and a portion of the surface of the conductor roll 5 is exposed.
If there is a delayed start or an early end of the electrolytic removal of the deposited metal, on the other hand, the electroplating metal deposited onto the surface of the conductor roll 5 causes in the meantime an insufficient electrical contact between the conductor roll 5 and the steel strip 1, and the electroplating metal deposited onto the surface of the conductor roll 5 is peeled off and falls down onto the steel strip 1 and produces flaws on the surface of the steel strip 1.
Under such circumstances, there is a strong demand for the development of an apparatus for properly removing an electroplating metal deposited onto the surface of a conductor roll, the lower portion of which is immersed into an electroplating solution in a horizontal type electroplating tank, without causing any electrolytic erosion of the conductor roll itself, but such an apparatus has not as yet been proposed.